1. Field of the Invention
The present invention relates to an electric-power steering controller, and more particularly to cutting off power supply when failure occurs.
2. Description of the Related Art
In FIG. 2, the circuit configuration of a conventional electric power steering controller is illustrated. In such an electric power steering controller, a motor M is adapted to provide an unillustrated steering system with assist torque. In order to control an electric power steering, a microcontroller (hereinafter referred to as MCU) 1, using a motor-current target value calculated based on output from an unillustrated torque sensor, a vehicle speed sensor, or the like, and using a motor-current detected value detected through a shunt resistor 2, acquires a target voltage to be applied and outputs to a gate driver 3 a controlling signal corresponding to the target voltage to be applied. The gate driver 3 controls the rotating direction and the output torque of the motor M by switching on and off, by means of a driving signal based on the controlling signal, each of switching devices constituting a bridge circuit 4, and by controlling a voltage to be applied to the motor M.
In such a conventional electric power steering controller, if MCU 1 has determined that any failure exists in the electric power steering controller, MCU 1 stops driving the motor by cutting off a power supply relay 6 provided between the bridge circuit 4 and a battery 5, or a motor relay 7 provided between the bridge circuit 4 and the motor M (refer to Japanese Laid-Open Patent Publication (Examined) 1995-96387). Because the methods for detecting the failure are commonly known, the description for those will be omitted.
However, in the controller as described above, it is necessary to insert the power supply relay 6 provided between the bridge circuit 4 and the battery 5, or the relay provided between the bridge circuit 4 and the motor M. During normal conditions, the relay has to let an electric-motor current flow; because a current of several tens of amperes flows in the case of an electric power steering apparatus, a sizable relay with a large current capacity is needed to be employed, resulting in an obstacle to downsizing the controller.
In addition, when disposed on a board or the like, a relatively sizable relay has its center of gravity far from (high off of) the mounting surface, whereby it is susceptible to various vibrations (engine vibration or vibration in driving) of vehicles equipped with an electric power steering apparatus therein; therefore, the connecting portions on the relay are liable to snap, and its contacts are liable to weld, resulting in an obstacle to the reliability hike of the controller.
The present invention addresses the foregoing problems, and its object is to provide an electric power steering controller in which a redundant system equal to that with a relay is configured, without incurring the upsizing, or deterioration in reliability, of the controller.